One danger is that they may interact with each other in a bad way if they’re taken within a couple of hours of each other. For example, taking magnesium will reduce zinc’s absorption, so you won’t be getting as much zinc as you think you are if you take the two at around the same time. Same goes for most fat-soluble vitamins like vitamin K and vitamin D.
Sometimes, a combination can cause serious medical problems. For example, taking the combination of St. John’s wort for depression and kava for anxiety may lead to serious liver problems, and also increase side effects. This is particularly concerning since depression and anxiety often co-exist, leading to an increased risk that someone who’s considering supplements would take both of these.
There’s also the risk of going overboard when it comes to vitamins and minerals. If someone is taking a multivitamin on top of other supplements or enriched food products containing single vitamins, they may get more than the recommended dose. Sometimes, this isn’t a huge problem. But sometimes it can be, like in the case of getting mega-doses of iron or niacin.
Mega-doses are a particular problem for supplements, because they’re not very well regulated, and you don’t always know whether the dose on the bottle matches what’s actually in the pill you’re taking. Sometimes you can get much more—or much less—than what’s on the label with each dose. And sometimes the dose can jump around depending on the batch.
Finally, because of the loose regulation of supplements, each supplement you take may also increase the risk of taking a contaminated substance. While contamination occasionally still happens with well-regulated pharmaceuticals, it’s a bigger concern for supplements. Sometimes shady manufacturers intentionally put drugs in supplements; this happens most frequently with sexual health, fat burner, and pre-workout blends.